SS 202 vs SS 304 vs SS 316 — Which Stainless Steel Grade to Use
There is a massive misconception in the manufacturing world that all "stainless steel" is created equal. Buying a generic "stainless" bolt for a marine application or a high-temperature industrial oven is a guaranteed path to failure. Stainless steel is an alloy family, not a single material, and standardizing on the wrong grade will either bankrupt your project or cause catastrophic corrosion over time.
If you are building an outdoor robotics project, a custom 3D printer enclosure, or assembling industrial IoT hardware, understanding the precise differences between SS202, SS304 (A2), and SS316 (A4) is non-negotiable. Here's exactly what you are paying for when you buy these fasteners.
The Secret is Molybdenum
If you are dealing with salt, seawater, or severe chlorides, you need Molybdenum. SS316 has it. SS304 does not. That one element is the difference between a 20-year lifespan and a 6-month failure.
The Core Breakdown: 202 vs. 304 vs. 316
| Grade | Also Known As | Key Elements | Corrosion Resistance | Best For |
|---|---|---|---|---|
| SS 202 | A1 (Rarely) | Chromium, Manganese, Nitrogen (Low Nickel) | Low. Will rust quickly outside or in damp conditions. | Indoor use only. Cheap consumer goods, indoor brackets, temporary fixtures. |
| SS 304 | A2, 18/8 Stainless | 18% Chromium, 8% Nickel | High. Excellent for fresh water and weather exposure. | General engineering, automotive trim, food processing equipment, high-end 3D printers. (The Industry Standard) |
| SS 316 | A4, Marine Grade | 16% Chromium, 10% Nickel, 2% Molybdenum | Extreme. Withstands saltwater, chlorides, and harsh chemicals. | Marine environments, chemical plants, offshore oil rigs, high-acidity applications. |
Why Are SS304 and SS316 Not Magnetic?
A common "test" for stainless steel is putting a magnet to it. If the magnet doesn't stick, people assume it's high quality. Wait!
SS304 and SS316 are Austenitic stainless steels. Because of their high Nickel content, their crystalline structure changes in a way that makes them fundamentally non-magnetic. However, cold-working a bolt (rolling the threads under extreme pressure) can locally alter the microstructure, making the threads slightly magnetic. Do not assume your A2/SS304 bolts are "fake" just because a strong neodymium magnet slightly pulls on the threads.
SS202: The Budget Trap to Avoid
Many cheap fasteners imported into the market are quietly made of SS202. Because Nickel is extremely expensive, SS202 replaces much of the Nickel with Manganese and Nitrogen. It looks identical to SS304 and polished perfectly, but its pitting resistance is terrible.
RPI Shop Standard: We do not recommend SS202 for any critical or outdoor engineering application. If your aluminum extrusion frame will ever see moisture or variable weather, relying on SS202 sliding T-nuts or bolts will lead to galvanic corrosion locking the bolt into the extrusion forever.
Making the Right Choice
If your project is largely indoors, inside an electronics enclosure, or in a dry environment: SS304 (A2) is the most cost-effective, high-quality choice you can make. It is the gold standard for mechanical assemblies.
If your project will be mounted on a boat, submerged in pool water, exposed to road salt in winter, or operating in a chemical factory: Do not hesitate. Pay the premium for SS316 (A4). The cost of replacing failed fasteners later will infinitely eclipse the extra cost of A4 bolts today.